The present invention relates to an exhaust gas recirculation system for recirculatingly using part of exhaust gas from an engine so as to purify the exhaust gas.
Exhaust gas recirculation is known as one solution for purifying exhaust gas from an engine of a vehicle such as automobile. In such exhaust gas recirculation, part of the exhaust gas is taken out of an exhaust system of the engine, is returned to a suction system and is added to the air-fuel mixture. This increases the ratio of inert gas such as H.sub.2 O, N.sub.2 and CO.sub.2 in a combustion air-fuel mixture and decreases combustion temperature so that generation of NO.sub.x is suppressed. However, when excessive quantity of exhaust gas is recirculated, combustion becomes unstable and both HC level and fuel consumption increase. Therefore, it is necessary to control exhaust gas recirculation quantity within a certain limit at which a desired NO.sub.x level and engine stability are compatible.
FIG. 1 shows an example of the above-mentioned exhaust gas recirculation applied to an engine with a turbocharger (Japanese Utility Model 1st Publication or Laid-Open No. 1-173445). More specifically, in a turbocharger 1 with a turbine 2 and a compressor 3 integrated with each other through a bearing casing 4, the turbine 2 is connected to an exhaust manifold 6 which serves as an exhaust passage of the engine 5. The turbine 2 is driven by exhaust gas from the engine 5 to drive the compressor 3 where compressed suction air is supplied through a suction passage 7 to the engine 5. A circulation passage 8 is provided between the exhaust manifold 6 and a downstream portion of the suction passage 7 so as to pass part of the exhaust gas G from the engine 5 through the circulation passage 8 to the suction passage 7 for recirculation. Further, control valves 10 and 11 are provided respectively at an inlet and an outlet of the circulation passage 8 and are controlledly opened and closed by a controller 9.
In the above conventional system, the circulation passage 8 is connected at its outlet to the suction passage 7 where pressure has been increased by the compressor 3. Therefore, the exhaust gas sometimes flows in reverse direction depending upon pressure conditions, resulting in failure of properly joining the exhaust gas to the suction system.
To overcome this problem, it may be contemplated that the circulation passage 8 be connected to an upstream portion of the compressor 3. In this case, however, the compressor impeller will be contaminated to decrease performance of the turbocharger 1. Further, if there are any pulsations in the exhaust gas, pressure waves may be transmitted to the compressor impeller to cause resonance.
In view of the above, a primary object of the present invention is to provide an exhaust gas recirculation system in which part of exhaust gas from an engine having a turbocharger can be properly joined to a suction system without contaminating a compressor impeller and in which resonance of the compressor impeller is prevented even when there are any pulsations in the exhaust gas.
To attain the above object, in an exhaust gas recirculation system wherein a turbocharger with a turbine and a compressor integrated with each other is mounted on an engine, part of exhaust gas from the engine being joined back through a circulation passage to a suction system where air from the compressor is supplied to the engine, the present invention provides an annular chamber in a diffuser defining wall of said compressor, said chamber being connected with an outlet of said circulation passage, and a slit on said diffuser defining wall for communicating an inner periphery of said chamber with the diffuser.
A water-cooled jacket may be mounted on an outer surface of the diffuser defining wall.
The exhaust gas from the engine is introduced into the diffuser of the compressor having relatively low pressure so that the exhaust gas is introduced into the suction system without being hindered by compressor pressure. In this case, the compressor impeller is not contaminated since the diffuser is located downstream of the compressor impeller. Further, since the exhaust gas is passed to the chamber in the diffuser defining wall and then is introduced into the diffuser, any possible pulsation pressure in the exhaust gas is attenuated in the chamber and resonance of the compressor impeller is prevented.
When the water-cooled jacket is arranged on the diffuser defining wall, the chamber is cooled down to cool the exhaust gas passing through the chamber. Accordingly, the temperature of the gas passed to the engine can be decreased and this contributes to reduce the ratio of NO.sub.x content in the exhaust gas.